(a) Field of the Invention
The present invention relates to a cathode ray tube (CRT). More particularly, the present invention relates to a CRT having a scanning velocity modulation (SVM) coil, and to an electron gun applied to this CRT.
(b) Description of the Related Art
A CRT typically includes a panel, a funnel, and a neck, which are integrally fused to define an exterior of the CRT. A phosphor screen is formed on an interior surface of the panel. Also, an electron gun, which emits electron beams toward the phosphor screen, is mounted within the neck. The funnel is positioned between the panel and the neck, and has a deflection yoke mounted to an outer circumference thereof for deflecting the electron beams emitted from the electron gun.
A configuration in which an SVM coil is mounted on an outer circumference of the neck of the CRT is well known (e.g., the projection-type CRT). The SVM coil synchronizes the electron beams passing through electrodes of the electron gun with image signals applied to the CRT to deflect the electron beams, thereby improving the resolution around edges of the image realized on the phosphor screen. The SVM coil is generally comprised of two saddle-shaped coils that are connected in series.
FIG. 9 is a partial sectional view showing a conventional structure of a CRT having an SVM coil. As shown, the direction to the left in the drawing is the direction toward the electron gun, and the direction to the right in the drawing is the direction toward a panel.
An electron gun 1 includes a cathode 3 that emits electron beams. The electron gun 1 also includes a plurality of grid electrodes G1, G2, G3, G4, and G5 (hereinafter referred to as first, second, third, fourth, and fifth grid electrodes, respectively) that focus and accelerate the electron beams emitted from the cathode 3. A bead glass 5 aligns and fixes the first, second, third, fourth, and fifth grid electrodes G1, G2, G3, G4, and G5 in this sequence.
The first and second grid electrodes G1 and G2 have a short length in an axial direction Z of the CRT, while the third and fourth grid electrodes G3 and G4 are cylindrical and have a longer length in the axial direction Z relative to the first and second grid electrodes G1 and G2. The fourth grid electrode G4 acts as a focusing electrode that focuses electron beams. An SVM coil 7 is mounted to an outer circumference of a neck 9 at a position approximately corresponding to the location of the fourth grid electrode G4.
In the CRT structured as in the above, the SVM coil 7 applies a deflection magnetic field to the electron beams generated by the electron gun 1 so that the electron beams perform scanning in a favorable state to desired locations of a phosphor screen (not shown). However, with such a structure, the SVM magnetic field does not directly act upon the electron beams, and instead is partially blocked by the fourth grid electrode G4 such that its strength is reduced. Therefore, the electron beam position is unable to be precisely controlled.
Furthermore, an eddy current is generated on a surface of the fourth grid electrode G4 by the SVM magnetic field passing through the fourth grid electrode G4. This further weakens the magnetic field acting on the electron beams. The eddy current is directly proportional to a surface area of the electrode that blocks the magnetic field.
In an attempt to remedy these problems, Japanese Laid-Open Patent No. Showa 55-146847 discloses a CRT in which an electron gun corresponding to a position of an SVM coil is realized through at least two individual electrodes with a predetermined gap between the electrodes. The SVM coil is mounted to an outer circumference of a neck corresponding to a position of the gap (of the electron gun electrodes) such that a magnetic field generated by the SVM coil passes through the gap.
Although a sensitivity of the magnetic field can be enhanced with increases in gap size, such increases weaken the ability of the individual electrodes to focus electron beams as a result of an electrical field that enters from an exterior of the electrodes (e.g., an electrical field formed by a connector that electrically connects the individual electrodes). Therefore, there are limitations placed on how much the gap may be enlarged, and these limitations are such that the gap may not be made large enough to realize sufficient improvements in magnetic field sensitivity of the SVM coil. Stated differently, there are limits to how much the sensitivity of the magnetic field of the SVM coil can be increased.
Japanese Laid-Open Patent No. Heisei 2000-188607 discloses an electron gun in an effort to overcome these problems. In this patent, electrodes corresponding to a position of an SVM coil are formed in a coil configuration in an attempt to prevent a deterioration in the focusing of the electron beams by the influence of an exterior electric field, and to suppress eddy currents generated on the electrodes.
However, the coil-shaped electrodes are mounted along all directions perpendicular to an axial direction of the CRT such that although part of the magnetic field generated by the SVM coil acts on the coil-shaped electrodes, there is no substantial increase in the effect of the SVM coil with this configuration.